Process for preparing electrically conductive pigments

ABSTRACT

The invention relates to a process for preparing electrically conductive pigments based on F − - and/or PO 4   3− -doped tin mixed oxides applied as an electrically conductive layer to a substrate, in which, first of all, SnO 2 -coated substrates are prepared by precipitation and subsequent calcining and then, in further process steps, this SnO 2  layer is converted into a tin mixed oxide layer doped with F −  and/or PO 4   3− . The invention further relates to electrically conductive pigments prepared by the process of the invention, to the use of these pigments for pigmenting lacquers, printing inks, plastics systems or coatings, and to lacquers, printing inks, plastics systems or coatings pigmented with an electrically conductive pigment prepared by the process of the invention.

[0001] The invention relates to a process for preparing electrically conductive pigments based on F⁻- and/or PO₄ ³⁻-doped tin mixed oxides applied as an electrically conductive layer to a substrate. The invention further relates to electrically conductive pigments prepared by the process of the invention, to the use of these pigments for pigmenting lacquers, printing inks, plastics systems or coatings, and to lacquers, printing inks, plastics systems or coatings pigmented with an electrically conductive pigment prepared by the process of the invention.

[0002] In many sectors of industry there is a need for conductive pigments which can be used, for example, to produce plastics, lacquers, coatings or the like which are electrically conductive or antistatic or which screen against electromagnetic waves. Conductive carbon black is used in large amounts for this purpose and yet because of its high light absorption in the visible spectral region cannot be used for transparent, pale or coloured coatings. A further problem is the strong absorption of carbon black in the IR region, which in the case, for example, of solar irradiation leads to frequently unwanted warming of the coated articles.

[0003] For pale, electrically conductive coatings, therefore, nickel-coated graphite, metal flakes and mixed oxides, such as antimony-doped tin dioxide (SnO₂), for example, are being used. The mixed oxides mentioned may have been applied to supports, for example to platelet-shaped mica or spherical barium sulfate. The use of a mixed oxide in this way as an electrically conductive layer on a substrate has the twin advantages firstly that the behaviour of the pigments in coating systems and on other polymeric layers is improved and secondly that it is possible to reduce the price of these systems.

[0004] U.S. Pat. No. 4,431,764 describes, for example, transparent, electrically conductive coatings comprising a film-forming binder and finely divided tin oxide doped with from 0.1 to 20% by weight of antimony in the form of Sb₂O₃ or Sb₂O₅. EP-A-0 375 575 discloses conductive platelet-shaped pigments comprising as conductive layer an antimony-doped tin oxide, with a thin silicon dioxide layer arranged between the conductive layer and the substrate. The application of an additional layer to the substrate, however, significantly increases the complexity of the preparation process and therefore makes the pigment more expensive. Furthermore, antimony-containing tin oxides possess the disadvantage that they have a blue coloration to a greater or lesser extent depending on the antimony content and calcining temperature. A further factor is that tin oxides containing antimony oxide, and substances containing antimony oxide in general, do not appear to be unobjectionable from a toxicological viewpoint.

[0005] JP 60-223167 and JP 62-050344 describe mica platelets and kaolin platelets coated with indium-tin oxide (ITO) and featuring relatively high transparency and relatively good electrical conductivity. The disadvantage of these pigments, however, lies in the relatively high price of indium. DE-A-43 33 673 describes electrically conductive pigments in which the dopants used include, instead of antimony oxide, oxides of aluminium. The disadvantage of these mixed oxides, however, lies in the low temporal stability of the resistance of these pigments.

[0006] In order to obtain electrically conductive pigments with a high and temporally stable electrical resistance, therefore, DE-A-198 11 694 doped tin mixed oxide with fluoride (F⁻) and/or phosphorus (P⁵⁺) by reacting hydrolysable tin(II) and tin(IV) compounds in water in the presence of one or more fluorides and/or phosphorus compounds. The doped mixed oxides thus obtained were applied as an electrically conductive layer to a substrate.

[0007] The preparation process by which these F⁻- and/or P⁵⁺-doped electrically conductive layers applied to a substrate were prepared is, however, comparatively expensive and complex. Another possibility for preparing electrically conductive pigments based on F-and/or P⁵⁺-doped tin mixed oxides is to grind SnO₂, SnO and also F⁻ and/or PO₄ ³⁻ using high mechanical force and then to calcine the ground pigments under a nitrogen atmosphere. In this way, although it is possible to obtain electrically conductive pigments having a resistance of between 200 and 300 Ω·cm, these products have a high surface roughness and are therefore of only limited usefulness in polymer solutions, such as coating materials. Moreover, the use of these products as an electrically conductive layer on substrates is not possible by this method.

[0008] Accordingly, it was an object of the present invention to provide a process for preparing electrically conductive pigments based on F⁻ and/or PO₄ ³⁻-doped tin mixed oxides applied as an electrically conductive layer to a substrate which is comparatively simple and inexpensive. This process should nevertheless lead to electrically conductive pigments which possess acceptable conductivities and in which the aforementioned disadvantages are at least present to a lesser extent.

[0009] It has surprisingly now been found that electrically conductive pigments based on F⁻ and/or PO₄ ³⁻-doped tin mixed oxides applied as an electrically conductive layer to a substrate may be prepared by first preparing SnO₂-coated substrates, in a simple and inexpensive process, and by then converting this SnO₂ layer into a tin mixed oxide layer doped with F⁻ and/or PO₄ ³⁻, in further process steps.

[0010] The present invention accordingly provides a process for preparing electrically conductive pigments based on F⁻ and/or PO₄ ³⁻-doped tin mixed oxides applied as an electrically conductive layer to a substrate. In this process, first of all, a substrate coated with tin(IV) oxide hydrate is prepared by adding a hydrolysable tin(IV) compound to an aqueous solution containing the substrate in suspended form, at a pH and temperature which permit complete hydrolysis of the tin(IV) compound, and this substrate is separated from the suspension and, if desired, washed and dried, and is subsequently calcined at a temperature between 600 and 1000° C. to form a substrate coated with SnO₂.

[0011] The hydrolysable tin(IV) compound is preferably used in an amount such that the SnO₂, after calcining of the tin(IV) oxide hydrate-coated substrate, possesses a weight fraction of from 5 to 200% by weight, preferably from 30 to 100% by weight, with particular preference from 40 to 60% by weight, based on the weight of the substrate.

[0012] Substrates which can be used include all water-insoluble inorganic compounds, preferably in the form of platelet-shaped particles or else spherical particles. Preferred substrates used comprise mica, glass flakes, talc, graphite, Al₂O₃, BaSO₄, ZnO, SiO₂ or else a mixture of these substances. These particles preferably have an average diameter of less than 150 μm and, with particular preference, not more than 100 μm. Platelet-shaped substrates preferably have an extent in the main dimension of less than 150 μm and, with particular preference, less than 100 μm, and their thickness is preferably less than 10 μm, with particular preference not more than 2 μm. The ratio of the extent in the main dimension to the thickness (aspect ratio) in the platelet-shaped substrates is preferably more than 3 and with particular preference more than 5.

[0013] In further process steps, the SnO₂ layer is subsequently converted into a tin mixed oxide layer doped with F⁻ and/or PO₄ ³⁻.

[0014] In one preferred embodiment of the process of the invention, this conversion of the SnO₂ layer into a tin mixed oxide layer doped with F⁻ and/or PO₄ ³⁻ takes place by first reducing the SnO₂-coated substrate with a reducing agent to form a substrate coated with an SnO₂/SnO/Sn mixed oxide. The reducing agent used in this case is preferably hydrogen or silicon. The substrate coated with this tin mixed oxide is subsequently ground with salts or salt mixtures containing F⁻ and/or PO₄ ³⁻ and finally is calcined in the absence of oxygen at a temperature of between 300 and 900° C. This gives a substrate which is coated with a tin mixed oxide layer doped with F⁻ and/or PO₄ ³⁻. The salts or salt mixtures used that contain F⁻ and/or PO₄ ³⁻ are preferably NaF, NH₄F, Na₂HPO₄, (NH₄)₂HPO₄ or mixtures of these salts. These salts or salt mixtures are ground with the substrates coated with an SnO₂/SnO/Sn mixed oxide preferably in an amount such that the salts or salt mixtures possess a weight fraction of from 1 to 15% by weight, with particular preference from 2 to 10% by weight, with very particular preference from 4 to 8% by weight, based on the weight of the substrate coated with the SnO₂/SnO/Sn mixed oxide.

[0015] In a further, preferred embodiment of the process of the invention, the conversion of the SnO₂ layer into a tin mixed oxide layer doped with F⁻ and/or PO₄ ³⁻ takes place by grinding the SnO₂-coated substrate with SnF₂ and/or SnHPO₄ and finally calcining it in the absence of oxygen at a temperature of between 300 and 900° C. This gives a substrate which is coated with a tin mixed oxide layer doped with F⁻ and/or PO₄ ⁻. The SnO₂-coated substrate is ground with SnF₂ and/or SnHPO₄ preferably in an amount such that SnF₂ and/or SnHPO₄ possess a weight fraction of from 1 to 15% by weight, with particular preference from 2 to 10% by weight, with very particular preference from 4 to 8% by weight, based on the weight of the SnO₂-coated substrate.

[0016] In the process of the invention it is preferred to use F⁻ and PO₄ ³⁻ for doping. The ratio of F⁻ to PO₄ ³⁻ is preferably between 1:2 and 2:1.

[0017] In comparison to the precipitation of hydrolysable tin(II) and tin(IV) salts in the presence of F⁻ and/or PO₄ ³⁻, the process of the invention is less complex and, moreover, less expensive, and yet gives electrically conductive pigments having acceptable resistances.

[0018] The invention further relates to electrically conductive pigments prepared by the process of the invention.

[0019] The invention additionally relates to the use of electrically conductive pigments prepared by the process of the invention for pigmenting lacquers, printing inks, plastics systems or coatings.

[0020] Finally, the invention relates to lacquers, printing inks, plastics systems or coatings pigmented with an electrically conductive pigment prepared by the process of the invention.

WORKING EXAMPLES Preparation of the SnO₂-coated Substrates

[0021] 100 g of mica are suspended in 2.5 liters of deionized water and the suspension is heated to 75° C. The pH of this suspension is adjusted to 2.1. Subsequently, 75 ml of an aqueous solution of SnCl₄ are added dropwise at a rate of from 0.5 to 1 ml/min with a stirring speed of from 400 to 1000 rpm. After all of the SnCl₄ has been added, the substrate coated with tin(IV) oxide hydrate is separated off, washed and dried at from 120 to 140° C. The dried product is subsequently calcined at from 600 to 1000° C. The resistance of the SnO₂-coated substrate obtained is less than 1000 kΩ·cm.

Example 1 Preparation of the Electrically Conductive Pigments

[0022] 40 g of the SnO₂-coated substrate are reduced in an H₂/N₂ gas stream (15%/85%) at 600° C. for 30 minutes. The resistance of the substrate coated with an SnO₂/SnO/Sn mixed oxide at this point in time is approximately 1 kΩ·cm. 20 g of the substrate coated with the SnO₂/SnO/Sn mixed oxide are subsequently ground with 1 g of NaF and 1 g of Na₂HPO₄ and calcined under a nitrogen atmosphere at 600° C. for 30 minutes. This gives a pale grey pigment powder. The resistance of the SnO₂/SnO/Sn mixed oxide obtained in this way, doped with F⁻ and PO₄ ³⁻ and applied to a substrate, is 350 Ω·cm.

Example 2 Preparation of the Electrically Conductive Pigments

[0023] 20 g of the SnO₂-coated substrate are ground with 1 g of SnF₂ and 1 g of SnHPO₄ and calcined under a nitrogen atmosphere at 600° C. for 30 minutes. This gives a pale grey pigment powder. The resistance of the SnO₂/SnO mixed oxide thus obtained, doped with F⁻ and PO₄ ³⁻ and applied to a substrate, is 300 Ω·cm. 

1. Process for preparing electrically conductive pigments based on F⁻- and/or PO₄ ³⁻-doped tin mixed oxides applied as an electrically conductive layer to a substrate, characterized in that, first of all, a substrate coated with tin(IV) oxide hydrate is prepared by adding a hydrolysable tin(IV) compound to an aqueous solution containing the substrate in suspended form, at a pH and temperature which permit complete hydrolysis of the tin(IV) compound, and this substrate is separated from the suspension and, if desired, washed and dried, and is subsequently calcined at a temperature between 600 and 1000° C. to form a substrate coated with SnO₂, and in that, subsequently, in further process steps, this SnO₂ layer is converted into a tin mixed oxide layer doped with F⁻ and/or PO₄ ³⁻.
 2. Process for preparing electrically conductive pigments according to claim 1, characterized in that the conversion of the SnO₂ layer into a tin mixed oxide layer doped with F⁻ and/or PO₄ ³⁻ takes place by first reducing the SnO₂-coated substrate with a reducing agent to form a substrate coated with an SnO₂/SnO/Sn mixed oxide and subsequently grinding the substrate coated with this tin mixed oxide with salts or salt mixtures containing F⁻ and/or PO₄ ³⁻ and finally calcining the ground pigment in the absence of oxygen at a temperature of between 300 and 900° C.
 3. Process for preparing electrically conductive pigments according to claim 2, characterized in that the reducing agent used is water or silicon.
 4. Process for preparing electrically conductive pigments according to claim 2 or 3, characterized in that the salts or salt mixtures used that contain F⁻ and/or PO₄ ³⁻ are NaF, NH₄F, Na₂HPO₄, (NH₄)₂HPO₄ or mixtures of these salts.
 5. Process for preparing electrically conductive pigments according to claims 2 to 4, characterized in that the substrate coated with an SnO₂/SnO/Sn mixed oxide is ground with salts or salt mixtures containing F⁻ and/or PO₄ ³⁻ in an amount such that the salts or salt mixtures possess a weight fraction of from 1 to 15% by weight based on the weight of the substrate coated with the SnO₂/SnO/Sn mixed oxide.
 6. Process for preparing electrically conductive pigments according to claim 1, characterized in that the conversion of the SnO₂ layer into a tin mixed oxide layer doped with F⁻ and/or PO₄ ³⁻ takes place by grinding the SnO₂-coated substrate with SnF₂ and/or SnHPO₄ and finally calcining it in the absence of oxygen at a temperature of between 300 and 900° C.
 7. Process for preparing electrically conductive pigments according to claim 6, characterized in that the SnO₂-coated substrate is ground with SnF₂ and/or SnHPO₄ in an amount such that SnF₂ and/or SnHPO₄ possess a weight fraction of from 1 to 15% by weight based on the weight of the SnO₂-coated substrate.
 8. Process for preparing electrically conductive pigments according to one or more of claims 1 to 7, characterized in that the hydrolysable tin(IV) compound is added to the substrate suspension in an amount such that the SnO₂, after calcining of the tin(IV) oxide hydrate-coated substrate, possesses a weight fraction of from 5 to 200% by weight based on the weight of the substrate.
 9. Process for preparing electrically conductive pigments according to one or more of claims 1 to 8, characterized in that the substrate used comprises mica, SiO₂ flakes, talc, graphite, Al₂O₃, BaSO₄, ZnO, SiO₂ or a mixture of these substances.
 10. Process for preparing electrically conductive pigments according to one or more of claims 1 to 9, characterized in that the mixed oxide on the substrate is doped with F⁻ and PO₄ ³⁻, the ratio of F⁻ to PO₄ ³⁻ being between 1:2 and 2:1.
 11. Electrically conductive pigments prepared by a process according to claim
 1. 12. Use of electrically conductive pigments prepared by a process according to claim 1 for pigmenting lacquers, printing inks, plastics systems or coatings.
 13. Lacquers, printing inks, plastics systems or coatings pigmented with an electrically conductive pigment prepared by a process according to claim
 1. 